The present invention relates to the manufacture of chopped glass pellets for injection molding of reinforced polymers. In particular, the present invention provides sizing compositions and a process for the manufacture of glass fiber pellets having enhanced processability without any loss in dispersability characteristics of the glass pellets.
Sizing compositions are well known and widely used in the manufacture of glass or carbon fibers to improve their processing properties, such as: fiber bundle cohesion, bundling, spreadability, resistance to fuzz formation, fiber smoothness and softness, abrasion resistance and easy and nondestructive unwindability of bobbined fiber bundles. Sizing compositions also affect the physical properties of the composite containing the treated fibers.
The reinforced plastic industry has been using glass fibers in various forms for reinforcing polymeric matrices to produce a variety of products. Glass fibers have been used in the form of continuous or chopped filaments, strands and rovings, as well as woven and nonwoven fabrics, meshes and scrims to reinforce polymers. Thermoplastic polymeric matrices have been reinforced with a variety of different forms of glass fibers resulting in the production of products such as: sheet molding compounds, bulk molding compounds, pultrusion products, panel products, spray up molding products, etc.
Production of glass fibers for the polymeric reinforcement market involves, attenuation of the glass fibers from molten streams of fiberizable glass material from a bushing or like device connected to a furnace containing molten fiberizable glass material. The glass fibers are attenuated by conventional means such as winders or high pressure air jets. In the process of producing glass fibers, a chemical composition is applied to them shortly after they are attenuated as the molten streams of glass. Prior to the present invention, the chemical composition has traditionally been an aqueous solution, foam or gel composition containing film forming polymeric materials, coupling or keying agents, lubricants and sometimes processing aids. This chemical composition or sizing is necessary in order to retard inter filament abrasion of the glass fibers when they are gathered into a bundle of glass fibers or strands. It is also required in order to make the glass fibers compatible with polymer matrices that they are used to reinforce. After application of the sizing, the fibers are then dried either in the package form or in the chopped strand form before they are used for reinforcement.
Prior to the present invention, the next step in using glass fibers as reinforcement for molded polymers involved production of either a short fiber composite or a long fiber composite. In general, the production of short fiber composites involved mixing pure polymer pellets with the chopped glass fibers such that the glass fibers were dispersed throughout the polymer when extruded. Pultrusion is used to produce long fiber composites where hot, thermoplastic polymer is forced through the glass roving so as to make the composite. This process method for manufacturing the glass polymer composite is expensive and very slow mainly due to high viscosity of thermoplastic polymer.
As discussed above, the chopped glass fibers are commonly used as reinforcement materials in thermoplastic articles. Typically, such fibers are formed by pulling molten glass into filaments through a bushing or orifice plate, applying a sizing composition containing lubricants, coupling agents and film forming binder resins to the filaments, gathering the filaments into strands, chopping the fiber strands into segments of the desired length and drying the sizing composition. These chopped strand segments are thereafter mixed with a polymerizable resin, and the mixture is supplied to a compression or injection molding machine to be formed into glass fiber reinforced plastic articles. Typically, the chopped strands are mixed with pellets of a polymerizable thermoplastic resin, and the mixture is supplied to an extruder wherein the resin is melted, and mixed with the chopped strands thus, the integrity of the glass fiber strands is destroyed and the fibers are dispersed throughout the molten resin, the fiber length is decreased and the fiber/resin dispersion is formed into pellets. These pellets are then fed to the molding machine and formed into molded articles having a substantially homogeneous dispersion of the glass fibers throughout.
Unfortunately, however, chopped glass fibers made via such processes are typically bulky and do not flow well. Consequently, such fibers are sometimes difficult to handle and have, on occasion, been problematic in automated processing equipment.
Most attempts for improving the process have been directed toward compacting the chopped strands. The work was aimed at improving flowability of the chopped strands which would presumably enable the use of automated equipment to weigh and transport the glass fibers for mixing with thermoplastic resins.
Such a process is disclosed in U.S. Pat. No. 4,840,755, wherein wet chopped strands are rolled, preferably on a vibrating carrier, to round the strands and compact them into denser cylindrically-shaped pellets. However, while the disclosed methods do tend to provide denser, more cylindrically-shaped pellets exhibiting better flowability, the disclosed methods and apparatus are undesirably limited in certain respects. For example, the pellet size and fiber content is generally limited by the size and number of fibers in the chopped strand. Although separated strands or loose filaments reportedly adhere to other strands during the rolling process, the process is designed to avoid multiple chopped strand segments from adhering together to form pellets containing more fibers than are present in a single chopped strand. Consequently, to obtain pellets having a suitable bulk density and a sufficient ratio of diameter to length to exhibit good flowability, the strand from which the segments are chopped usually must be formed from a large number of filaments. However, increasing the number of filaments required to be formed and combined into a single strand undesirably complicates the forming operation.
Although the disclosed pellets can be made by such diverse mixing processes, it has been discovered that many of such processes are either too inefficient to be used commercially, or cannot be adequately controlled to produce a uniform pellet product that provides the resulting composite article with strength characteristics comparable to those made from nonpelleted chopped strand fibers. For example, the use of a modified disk pelletizer as described in U.S. Pat. No. 4,840,755 frequently results in excessive residence time of the formed pellets within the mixer, which results in degradation of the pellets due to the abrasive nature of glass fiber pellets rubbing against one another. Such pellet degradation ultimately reduces the strength characteristics of the molded articles made therewith.
In an attempt to overcome these shortcomings, U.S. Pat. No. 5,578,535 discloses glass fiber pellets that are from about 20 to 30 percent denser than the individual glass strands from which they are made, and from about 5 to 15 times larger in diameter. These pellets are prepared by hydrating cut strand segments to a level sufficient to prevent filamentization but insufficient to cause the strand segments to agglomerate into a clump, and mixing the hydrated strand segments for a time sufficient to form pellets. Suitable mixing includes any process that will keep the fibers moving over and around one another and include processed described as tumbling, agitating, blending, commingling, stirring and intermingling. However, although the disclosed pellets can be made by such diverse mixing processes, it has been discovered that many of such processes are either too inefficient to be used commercially, or cannot be adequately controlled to produce a uniform pellet product that provides the resulting composite article with strength characteristics comparable to those made from nonpelleted chopped strand fibers. For example, the use of a modified disk pelletizer as described in the patent frequently results in excessive residence time of the formed pellets within the mixer, which results in degradation of the pellets due to the abrasive nature of glass fiber pellets rubbing against one another. Such pellet degradation ultimately reduces the strength characteristics of the molded articles made therewith.
In summary, the previous attempts to improve upon chopped glass fiber strand technology have mainly been directed at the short fiber lengths (about 6 mm) and lower glass contents (about 30%); in an effort to maintain dispersion of the glass fibers in the injection molded part. This has resulted in a trade off in processability. Along with the processing and dispersion requirements, the industry has also been attempting to alleviate environmental concerns by finding ways to minimize levels of VOCs while maintaining the physical properties of the fibers. This has given rise to the use of NEAT (nonaqueous elevated application temperature) sizings. With NEAT sizings, the resins are not emulsified or mixed with solvents, therefore, the VOCs are significantly reduced. In addition, in the present invention the coupling agents, or more particularly, the silanes are not mixed in water; this in some cases reduces hydrolization and may decrease the release of VOCs into the production environment. This is demonstrated in U.S. patent application Ser. No. 081885,882, assigned to Owens-Corning now U.S. Pat. No. 5,998,029
Accordingly, a need exists for a method of processing fibers with a sizing composition that adds processability to the NEAT sized glass fibers while preserving the excellent dispersability in the molded product. Not only is such a need fulfilled by the process and composition of the invention below, but the invention has the additional aspect of increasing the notched Izod strength of the molded parts while also allowing for higher glass contents.
The present invention provides a nonaqueous sizing composition and process that controllably yields highly processable chopped glass fibers that increase the notched Izod values of injection molded products reinforced with the pellets. The resulting glass pellets can be used in the manufacture of any long fiber product requiring high properties. For example, the product of the invention may be used in injection and compression molded parts useful in the automotive industry, such as the internal parts of a car, without any loss in physical characteristics while being more processable.
Prior to the discovery of the present sizing composition, it was necessary to keep the molecular weight of the sizing low in order to cut down on drag. This resulted in problems of maintaining pellet integrity. However, the present invention allows for the addition of higher molecular weight materials which allows for the use of higher temperatures. This results in a chopped glass pellet having physical characteristics and processability comparable or better than those normally seen only in wirecoated pellets.
One embodiment of the nonaqueous sizing of the present invention contains one or more film formers which are miscible with the polymer to be reinforced and one or more coupling agents and one or more high molecular weight powders. The sizing contains no water and is applied at high temperatures.